The railroad industry, including but not limited to the freight railroad industry, employs wayside lights to inform train operators of various types of operational parameters. For example, colored wayside signal lights are often used to inform a train operator as to whether and how a train may enter a block of track associated with the wayside signal light. The status/color of wayside signal lamps is sometimes referred to in the art as the signal aspect. One simple example is three color system known in the industry as Automatic Block Signaling (ABS), in which a red signal indicates that the block associated with the signal is occupied, a yellow signal indicates that the block associated with the signal is not occupied but the next block is occupied, and green indicates that both the block associated with the signal and the next block are unoccupied. It should be understood, however, that there arc many different kinds of signaling systems. Other uses of signal lights to provide wayside status information include lights that indicate switch position, hazard detector status (e.g., broken rail detector, avalanche detector, bridge misalignment, grade crossing warning, etc.), search light mechanism (SA1) position, among others.
In recent years, there has been significant interest providing trains with what is sometimes referred to as PIC (positive train control) equipment in order to prevent collisions between trains. In the U.S., the use of such systems has been mandated by the Federal Railroad Administration in certain applications by 2015. There are many varieties of such PTC systems, but one thing many of them have in common is an onboard computer system that receives signal aspect information and ensures that the train complies with the received signal aspect information. Many of these systems also provide for the onboard receipt of additional information traditionally indicated by wayside lamps, such as switch position and the other types of information listed at the end of the previous paragraph.
In order for such systems to work properly, it is of course necessary for the signal aspect information and other wayside status information to be communicated to the equipment located onboard the train. It will be understood by those of skill in the art that there are many types of wayside signaling systems in use today, including CTC (centralized track control) and ABS (automatic block signaling) systems. In some of these systems, the signal aspects are controlled from a central location, whereas in other systems the signal aspects are controlled by relays in the field using, e.g., track circuits that detect the presence of a train in a block of track associated with the wayside signal device. The wayside signal aspect information from the latter types of system may be transmitted to a central office, but this signal aspect information is not vital and therefore cannot be relied on for the purpose of transmission to a PTC system onboard a train. One solution to the issues raised by the wide variety of wayside signaling systems in use today relies on a common denominator of such systems—the supply of current to signal lamps—by employing current sensors to sense such current in order to determine the wayside signal aspect. In order to use such a technique for the purpose of supplying signal aspect information and other types of wayside status information to an onboard PTC system, it is necessary for the current sensors to be vital.
Additionally, industry personnel often wish to ensure that the lamps in the wayside status information systems discussed above are working. They may do so by periodically changing the lamps, even if they are functioning properly, or by periodically testing the lamps either manually or with an automatic test system. The lamps in such devices can include incandescent lamps employing filaments, LEDs, and other types of lamps. For some types of lamps, wiring errors may allow the lamps to function properly, but may cause false readings in tests. Therefore it may be unclear from a test whether the lamp is working or not. These wiring errors may be hard to detect because the lamps function properly, so testing methods that work in spite of wiring errors are needed.